These real estate investors built cash-flowing portfolios. Here are the rules they swear by.

Jeff White and Suleyka Bolaños say renting individual rooms has helped them maximize rental income and cash flow.

Jeff White and Suleyka Bolaños Investors use rules like the 1% rule to screen for cash-flowing rentals.

Some boost cash flow by adding units: "The more doors on the land, the more money it produces." Others see mid-term rentals as the 'sweet spot' between long- and short-term leases.

Producing cash flow from rental property sounds simple: collect more in rent than you spend on the property.

In practice, getting the numbers to work can be much harder.

Investors have to find a property at the right price, finance it, account for ongoing expenses, and keep it occupied.

Business Insider has spoken with real estate investors across the country about the rules they use to find — and maintain — cash-flowing rentals.

Here are four that they swear by.

Use the 1% rule as a quick screen Ted Garber has specific cash-flow and return goals for each property he buys.

"Each rental needs to shed cash flow immediately and, on average, pay back our initial investment within three to six years," the Florida-based investor said.

One tool he uses to quickly evaluate a potential deal is the "1% rule," which suggests that a property's monthly rent should equal at least 1% of its purchase price.

For example, a $120,000 property would ideally rent for at least $1,200 a month.

Garber prefers an even wider margin.

On a $120,000 condo, for example, he'd rather collect closer to $1,500 a month to create a cushion for expenses.